1. Field of the Invention
This invention relates to a method and apparatus for obtaining tomographic information. This invention particularly relates to a method and apparatus for obtaining tomographic information, wherein a light beam scattered backwardly from a medium having light scattering properties is detected, and information representing the microstructure at the surface of the medium or at a portion deep from the surface of the medium is obtained from the backward scattered light beam.
2. Description of the Prior Art
Various methods and apparatuses for obtaining tomographic information have heretofore been proposed wherein tomographic information, such as a tomographic image, of a medium having light scattering properties, such as a living body, is obtained such that the medium may not be invaded.
As one of the methods for obtaining tomographic information, optical coherence tomography (hereinafter referred to as OCT) has heretofore been used. With the OCT, a low coherence light beam is split into a light beam, which is to be irradiated to a medium, and a reference light beam, and a Michelson type of interferometer is constituted by the two split light beams. Optical heterodyne detection is carried out on the interference light beam, which is obtained from the interferometer, and the intensity of a light beam scattered backwardly from the medium is thereby detected. From the detected intensity of the backward scattered light beam, the tomographic information at the surface of the medium or a deep portion in the medium is obtained. Tomographic information at an arbitrary deep potion in the medium can be obtained by modulating the optical path length of the reference light.
However, with the OCT, the coherence length of the low coherence light beam is short. Therefore, in cases where the medium, the tomographic information of which is to be obtained, is very large and must therefore be located at a position away from the position at which the low coherence light beam is split into the light beam to be irradiated to the medium and the reference light beam, it is necessary for the reference light beam to follow a long optical path from the position at which the two light beams are split from each other. Accordingly, the problems occur in that, in order for the variability of the optical path length to be kept large, the size of the apparatus for obtaining tomographic information cannot be kept small.
As a technique for solving the problems described above, a technique, wherein a reference light beam and a light beam to be irradiated to a medium are respectively guided through different single mode optical fiber bundles, has been proposed by David Huang, et al. in "Science," 1991, Vol. 254, pp. 1178-1181. With the proposed technique, a frequency shifting mechanism for shifting the frequency of the reference light beam is located in the optical path of the reference light beam, and a heterodyne signal is thereby obtained. Also, a mechanical mechanism for quickly displacing the leading end of the optical path of the light beam to be irradiated to the medium, i.e. the leading end of the single mode optical fiber bundle, along the optical axis direction is provided, and the information in the depth direction in the region inside of the medium is thereby obtained.
However, with the conventional technique described above, the leading end of the single mode optical fiber bundle must be displaced quickly by the mechanical mechanism. In addition, the single mode optical fiber bundle, through which the reference light beam passes, and the single mode optical fiber bundle, through which the light beam to be irradiated to the medium passes, are independent of each other. Therefore, the two single mode optical fiber bundles are liable to be affected in different manners by external vibrations or thermal disturbance. Accordingly, the frequency of a beat signal generated from the interference between the light beam, which has been irradiated to the medium and has thereafter been scattered backwardly from the medium, and the reference light beam, the frequency of which has been shifted, is affected in an unexpected manner by the external vibrations or thermal disturbance. In such cases, a signal, which has a low signal-to-noise ratio (S/N ratio), is obtained from the optical heterodyne detection.
Also, the conventional technique described above has the problems in that a long time is required for the two-dimensional scanning of the leading end of the single mode optical fiber bundle. In cases where image optical fiber bundles (ordinarily, multimode image optical fiber bundles) are employed in order to solve such problems, there is the risk that mode coupling occurs with each of the light beams, which are guided through the respective optical fiber bundles, due to external vibrations or thermal disturbance applied independently to the respective optical fiber bundles. A light beam resulting from superposition of the light beams, which have suffered from different mode coupling during the passage through the respective optical fiber bundles, one upon the other will have no significant effect.